The reason I asked in the Russell Holtz book, he states that "The closer the wings are to the ground, the more the ground interferes with the development of the wing tip vortices..."

On Thursday, December 15, 2016 at 8:45:06 PM UTC-8, Michael Opitz wrote:
At 03:23 16 December 2016, Dan Daly wrote:
On Thursday, December 15, 2016 at 10:12:12 PM UTC-5,
Jonathan St. Cloud
wrote:
Not being an aerodynamicist, I was wondering if winglets
decreased ground
effect. So would an ASW-20 (without winglets) be able to glide
further in
ground effect than an ASW-20 (with winglets), all else being
equal?

I googled it and this website answers the question
https://www.quora.com/Do-aircrafts-w...ts-experience-
lesser-ground-effect
. Not about the ASW-20 specifically.

It is on the internet, and therefore, must be true.

Winglets are a way to get most of the benefits of increasing the
span without actually increasing the footprint. The big advantage
for existing aircraft being retrofitted is that winglets don't produce
lift per se, so there is not much change to the bending moments,
therefore the spars don't have to be beefed up as they would for a
straight wing extension. (See the 15-18 m discussion on RAS.) For
airliners with a limited footprint available at gates, winglets allow an
increase in performance with retention of useage of the same gates.
Most general aviation winglets are specifically optimized for that
particular aircraft's cruise speed.

For gliders that have a span limit in their class, winglets are a
(relatively) cheap and easy way to increase performance. The
early glider winglets were generally good at increasing climb
performance, but hindered at the high speed end. The newer
generation of Maughmer type winglets are optimized to perform
much better over a broader speed range in gliders.

So, an ASW-20 with winglets is really like a ~16 meter ASW-20 at
altitude. The winglets probably reduce the ground effect
somewhat, but do they reduce it to a level less than a regular 15m
ASW-20? My guess is that the effects are probably too small for a
pilot to really notice a difference. It might be an interesting
Master's degree project for some grad student who has access to
the wind tunnel time to test the theory out....

There is a whole category of aircraft designed to operate in ground
effect in order to boost their range/payload. They are called WIG
(Wing In Ground effect) aircraft. Some have winglets which are
also pontoons going down, and others have winglets that go up at a
~45 degree angle. I guess the aerodynamic winglet/ground effect
reductiuon question is not settled with these aircraft either. See:
https://www.google.com/search?
q=WIG+aircraft&rlz=1C1CHWA_enUS602US603&espv=2&biw =128
0&bih=918&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahU KEwj
Z0auP7vfQAhVBKGMKHbCTCvsQsAQIMQ#imgrc=_

RO